I have acquired a Model C adapted to take a Graphic 23 back. So, normally I would use the ground glass back to check the placement of the infinity stops--except I don't have said gg/hood setup post-modification. So, now what? Also, the 23 has no dark slide--if I don't want to remove the back mid-roll, how likely are light leak issues?
It has the Raptar 101mm lens on its lens board._________________Stop me before I gear up again!

Open the roll film holder and remove the insert as if loading a roll of film. Place a piece of ground glass in the film plane that sits against the top and bottom guide rails that the pressure plate sit on.
There are two guide rails top and bottom. You want to use the outer most as the inner ones closest to the film opening are .010 lower than the outer ones.
Alternative: run scotch magic transparent tape, the frosted stuff that goes clear when pressed, from the top to the bottom of the film opening. Do not overlap. Focus on it as you would a piece of ground glass.

Using the outer rails will require you add a spacer to the pressure plate of the roll film holder if you set infinity with a ground glass focus frame on the camera and that ground glass is set to the same distance as the emulsion of a piece of film in a film holder._________________The best camera ever made is the one that YOU enjoy using and produces the image quality that satifies YOU.

I had a feeling that might be the method. I am going to take a guess that the cam is correct for the lens--and that once I confirm the position of the infinity stops, I can adjust the vernier on the rails as well. Did the blind squirrel (me) find the acorn?_________________Stop me before I gear up again!

Focusing cams for a factory marked focal length may be in error of 1 to 5mm focal length. An incorrect cam will be off at closer focused distances but be close enough at 25 feet to infinity to be usable on all but the fastest lens when used wide open.

Being the Pressman C is a 2x3 camera the darkslide from a 2x3 film pack holder might just work in the roll film holder._________________The best camera ever made is the one that YOU enjoy using and produces the image quality that satifies YOU.